Air conditioning systems which use a common electric motor driven fan to distribute the air throughout a building in both the heating and cooling modes are in wide use today. Usually, these systems are controlled from a thermostat which has a heat mode control switch and a cool mode control switch that alternatively operate the system in its heating mode or its cooling mode to condition the air in a building. Because such systems use a common fan motor, prior art controls for such systems have used a first fan control relay which operates the fan motor when the system is in its heating mode and a second relay which operates the fan motor when the system is in its cooling mode. Because of the cost of control relays, the use of two relays in such prior art systems has undesirably increased the manufacturing and installation cost of such systems and also increased the likelihood of system failure due to the malfunction of one of the fan control relays.